1. Field of the Invention
The present invention relates to a toner, and more particularly to a toner for developing an electrostatic latent image formed on an image bearer such as a photoreceptor to visualize the latent image. The present invention also relates to a developer including the toner and a container including the toner or the developer, and a method of producing the toner.
2. Discussion of the Background
A pressing and heating method with a heating roller in an electrophotographic image forming method passes a toner image on a transfer sheet through a surface of the heating roller having releasability with a toner while contacting the toner image upon application of pressure. The method can quickly fix a toner image on the transfer sheet because the surface of the heating roller contacts the toner image on the transfer sheet upon application of pressure and heat efficiency in fusion bonding of the toner image onto the transfer sheet is quite good. So-called offset phenomena, in which a part of a toner image adheres to a surface of a heating roller because the toner image melts and contacts the surface thereof upon application of pressure and the part of the toner image transfers onto a following transfer sheet and contaminates the sheet, is largely influenced by a fixing speed and temperature. Typically, when the fixing speed is slow, a surface temperature of the heating roller is set comparatively low, and when the fixing speed is fast, the surface temperature thereof is set comparatively high. This is to fix heat quantity imparted from the heating roller to the toner regardless of the fixing speed.
A toner on a transfer sheet has multiple layers, and particularly in an image forming apparatus in which a fixing speed is fast and a surface temperature of the heating roller is high, a temperature difference between a top layer contacting the heat roller and a bottom layer contacting the transfer sheet is large. When the surface temperature of the heating roller is high, the top layer of the toner tends to cause offset phenomena. When the surface temperature of the heating roller is low, phenomena called low-temperature offset tends to occur in which the bottom layer of the toner does not sufficiently melt and fix on the transfer sheet.
To solve this problem, a method of anchoring a toner on a transfer sheet by increasing the fixing pressure is usually used when the fixing speed is high. This method can decrease the heating roller temperature somewhat and prevent the high-temperature offset phenomena of the top layer of the toner. However, because a shearing force applied to the toner is quite large, the transfer sheet winds the heating roller and a winding offset occurs, and a trance of a separation pick separating the transfer sheet from the heating roller tends to appear on a fixed image. Further, the high pressure crushes a line image and causes toner scattering when fixed, resulting in deterioration of the fixed image quality.
In a high-speed fixing method, a toner having a lower melting viscosity than a toner used in a low-speed fixing method is typically used and a surface temperature and a fixing pressure of the heating roller are decreased to prevent the high-temperature offset and wind offset. However, when such a toner having a low melting viscosity is used in a low-speed fixing method, the high-temperature offset phenomena tend to occur.
As mentioned above, a toner having a good offset resistance and a wide range of fixable temperature, which is applicable in both a high-speed and a low-speed fixing methods, is desired.
On the other hand, a particle size of the toner is being downsized to increase resolution and sharpness of the resultant image. However, fixability of a halftone image formed with a small particle size toner deteriorates. This phenomenon noticeably occurs in high-speed fixing. This is because a toner amount is small on a halftone image, a toner transferred onto a concave portion of a transfer sheet receives less heat amount from a heating roller, and further a fixing pressure deteriorates as a convex portion of the transfer sheet inhibits the pressure to the concave portion thereof. As a toner layer transferred onto a halftone image on the convex portion of the transfer sheet is thin, a shearing force against a toner particle is larger than a shearing force against a toner particle on a black solid image having a thick toner layer. Therefore, the offset phenomena tend to occur and the resultant fixed image tends to be a low quality image.
Japanese Laid-Open Patent Publication No. 1-128071 discloses a toner including a polyester resin as a binder resin and having a specific storage viscosity at 95° C. However, fixability and offset resistance thereof still need to be improved.
Japanese Laid-Open Patent Publication No. 4-353866 discloses a toner having rheologic properties in which a drop starting temperature of a storage modulus is from 100 to 110° C., the toner has a specific storage modulus at 150° C. and a peak temperature of a loss elasticity is 125° C. However, as both the storage modulus and loss elasticity are too small and the peak temperature of the loss elasticity is too high, low-temperature fixability of the resultant toner is not improved and heat resistance thereof is low.
Japanese Laid-Open Patent Publication No. 6-59504 discloses a toner including a specifically structured polyester resin as a binder resin, and having a specific storage modulus at from 70 to 120° C. and a specific loss elasticity at from 130 to 180° C. However, as the storage modulus at from 70 to 120° C. is large and the loss elasticity at from 130 to 180° C. is small, a small particle size magnetic toner is difficult to fix at a low temperature and offset resistance of the toner of the invention is desired to be improved.
When the small particle size magnetic toner includes a large amount of a magnetic material, the fixability problem noticeably occurs. In terms of rheology, an increase of a colorant included in a toner tends to increase the storage modulus and loss elasticity. Therefore, when it is cold, a copy image produced immediately after a copier is switched on occasionally has worse fixability, which is desired to be improved.
Japanese Laid-Open Patent Publication No. 4-358159 discloses a developer including a vinyl polymer and two polyethylene and/or polyethylene waxes having different softening points, one of which is included in polymerizing and the other of which is included in kneading. As the two waxes have a high softening point of 100° C. and a small softening point difference of from 2 to 20° C., the resultant developer has good offset resistance, but poor low-temperature fixability.
Japanese Laid-Open Patent Publication No. 4-362953 discloses a toner including a de-free fatty acid carnauba wax and a rice wax oxide having an acid value of from 10 to 30. The toner has good low-temperature fixability, but low offset and blocking resistance and fluidity.
Japanese Laid-Open Patent Publication No. 6-130714 discloses a toner including a linear polyester as a fixing resin and waxes having a similar softening point to that of the linear polyester and a higher softening point than that thereof. The toner practically has good blocking resistance and offset resistance, but poor low-temperature fixability due to a high melting point of the waxes.
Japanese Laid-Open Patent Publication No. 11-133665 discloses a dry toner including an urethane-modified polyester as a toner binder and having a practical sphericity of from 0.90 to 1.00 for the purpose of improving fluidity, low-temperature fixability and hot offset resistance of the toner. In addition, the toner having a small particle size has good powder fluidity, transferability, thermostable preservability, low-temperature fixability and hot offset resistance. Particularly, when used in a full-color copier, the resultant image has good glossiness and an oil application to a heating roller is not required.
As a method of economically obtaining such a dry toner, Japanese Laid-Open Patent Publications Nos. 11-149180 and 2000-292981 disclose a dry toner and a method of producing the toner including a toner binder which is an elongation and/or a crosslinking reaction product of a prepolymer having an isocyanate group and a colorant, wherein the dry toner is a particulate material formed by an elongation and/or a crosslinking reaction of the prepolymer by amines in a water medium.
However, although a process using a urea reaction to prepare a binder in Japanese Laid-Open Patent Publication No. 11-133665 turns a new feature and an effect, the process is a pulverizing process and the resultant toner does not have sufficient low-temperature fixability. Specific conditions of controlling shapes of a small particle size and a sphere are not disclosed therein. In Japanese Laid-Open Patent Publications Nos. 11-149180 and 2000-292981, a toner is formed by granulation in water, wherein pigments in an oil phase agglutinate in a water phase interface, and a volume resistance thereof lowers and non-uniform dispersion thereof cause basic performance problems of the toner. An effect of the invention is not exerted without a targeted shape and properties to achieve oilless, small particle size and to be used in an apparatus. As the targeted shape and properties are not mentioned in respective publications, an effect of the invention is difficult to exert against problems. Particularly, pigments and waxes tend to gather on a surface of a toner formed by granulation in water, and when a particle size thereof is not greater than 6 μm, a specific surface area thereof becomes large. Therefore, a surface design of the particle is essential to obtain desired chargeability and fixability.
Because of these reasons, a need exists for a toner having good releasability, offset resistance, blocking resistance and fluidity as well as fixability.